chore(deps): update dependency xunit to 2.9.1

Author: renovate[bot]

tests/Prometheus.Client.MetricServer.Tests.csproj

@@ -6,7 +6,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
-    <PackageReference Include="xunit" Version="2.9.0" />
+    <PackageReference Include="xunit" Version="2.9.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>